A standard motor-vehicle bumper has a cross-wise bumper beam of omega section, that is having a normally vertical front web, upper and lower horizontal sides projecting inward toward the vehicle from upper and lower edges of the front web, and upper and lower flanges projecting upward and downward from inner edges of the sides. Such a cross beam is supported on the vehicle frame by at least two crash boxes each formed as a short horizontally extending tube having an outer end fixed to the cross beam and an inner end fixed to the vehicle frame, normally at a longitudinal rail. Thus in an accident the cross beam will be pushed back, crushing the crash boxes inward against the vehicle frame and thereby localizing damage to the crash boxes. Such a system is therefore intended to stop the vehicle in a low-speed 16 km/hr collision without significant damage to the vehicle frame or injury to its occupants, as crushing of the sacrificial crash boxes absorbs most of the energy of the collision.
It is often necessary as described in German 198 20 433 of R. Bruck, U.S. Pat. No. 6,364,384, and Japanese 6-211 091 for the crash box to also serve as an adapter to compensate for a difference in height between that of the vehicle frame, to which it must be centrally connected, and the bumper cross beam, whose height above the ground lies within a certain statutory range but may vary up or down somewhat for design considerations. Thus the inner end, that is the end secured to the vehicle frame, and the outer end of the crash boxes are often vertically offset from one another so that the bumper cross beam can be positioned above or below the center of the frame member to which the inner crash-box end is fixed.
A typical crash box is formed as described in German 100 57 311 of two squared-off U-section profiles that fit vertically together to form a rectangular tube. The box inner end is provided with a peripheral flange by means of which it is bolted to the vehicle frame. Such construction is a particular problem when the inner and outer ends are vertically offset as in a collision the horizontally applied force exerts a torque or twisting action on the crash boxes. The result is a lateral bending of the crash boxes instead of the desired crushing deformation, so that they absorb considerable less energy. Alternately the result is a twisting deformation of the vehicle frame, a part that the sacrificial crash box is supposed to protect.